So-called meterless hydraulic control circuits control the motion of a hydraulic actuator by controlling a flow from one chamber of the actuator to the other utilizing one or more pumps, that is, flow rate from the pump(s) is used to control the flow to and/or from the chambers of the actuator, as opposed to utilizing proportional valves. In metered systems, proportional or throttling valves are utilized to restrict or meter the fluid flow therethrough to control movement of the actuator. In contrast, in meterless systems, the pump(s) may be of a variable displacement type or of a fixed displacement type wherein the flow from the pump to the actuator chambers is varied in order to control the speed of the actuator movement.
In viewing the structure of an actuator, a rod extends from the one side of the piston and outward from the cylinder. As a result, the area of the piston on the side from which the rod extends is less than the area of the piston on the cap side of the actuator. Accordingly, the volumes of fluid displaced from the rod chamber and the cap side chamber differ. When extending the actuator, a supplemental volume of fluid is required in addition to the fluid displaced from the rod chamber. Conversely, when the actuator is retracted, the rod chamber cannot accommodate all of the fluid displaced from the cap side chamber. As a result, the hydraulic control circuits of this architecture include make-up circuits which provide and receive this excess hydraulic fluid.
In executing commanded motions that are in the opposite direction of the force applied by the load on the actuator, the pump acts as a pump, pumping fluid from one chamber to the other. Conversely, in executing commanded motions that are in the same direction the force applied by the load on the actuator, the load force acts to “push” the fluid from one chamber to the other such that the pump typically acts as a motor. In a motoring retraction, however, the volume of fluid flowing from the cap side chamber can be beyond the capabilities of the pump and driver, causing the pump to overspeed. Overspeeding can result in overheating the pump, or early pump failure if the pump is not adequately sized, which can result in higher costs.
One such meterless hydraulic control circuit is shown, for example, in U.S. Publication 2009/0165450. In this arrangement, two proposals are made for accommodating the excess fluid in a motoring retraction. In a first arrangement, fluid from the cap side chamber is directed through the pump and to the rod chamber; valves may be opened to direct a portion of the flow from the cap side chamber or the pump directly to the tank. In a second arrangement, all fluid from the cap side chamber is directed to the tank, and then fluid is pumped from the tank directly to the rod chamber.